The present apparatus relates to automotive exhaust systems, and more particularly relates to exhaust treatment devices such as catalytic converters and particulate traps that are vacuum-insulated and that have hydrides to variably control the thermal insulative value in the vacuum cavity and further that have getters to help preserve the vacuum for a longer period of time while in service.
Most vehicle exhaust systems and particularly exhaust systems of vehicles powered by internal combustion engines are equipped with catalytic converters for reducing noxious emissions in exhaust gases. Many vehicles such as those powered by a diesel engine also include particulate traps for reducing emission of particulates. A problem exists in that a large part of tailpipe emissions (such as HC, CO, NOx and/or particulates) occur during the initial cold start phase when the catalytic converter and particulate traps are least effective.
More specifically, modern engine exhaust treatment systems for vehicles include a catalytic converter which converts toxic exhaust emissions to non-toxic gases. The oxidative and reductive reactions that convert the emissions occur on the hot catalytically active surface of the converter. Until the converter is heated to a sufficiently high temperature, these reactions do not occur efficiently such that exhaust gases pass through the system untreated. EPA estimates indicate that as much as 80% of all automobile commuter exhaust emissions occur during the so-called “cold start” period when the catalytic converter is heating up to operational temperature.
A vacuum-insulated catalytic converter with included thermal energy storage improves the efficiency of engine exhaust emissions treatment by remaining hot long after the engine is shut off. If the engine is not shut off for too long, the still hot and catalytically active converter is immediately effective the next time the engine is used and avoids the “cold start” emission of untreated exhaust gases. The catalytic converter may also include a variable insulating system having an electrically heated, reversible source of hydrogen that communicates with the vacuum-insulated shell of the converter. This electrically controlled source of hydrogen provides a variable conductivity means of limiting the temperature of the converter so as to prevent it from overheating and suffering damage. Specifically, the hydrogen source captures hydrogen at low temperatures to maximize the insulative value of the vacuum cavity at the low temperatures, but releases hydrogen once the catalytic converter reaches a predetermined “light-off” temperature at which the catalytic reaction becomes exothermic. The presence of the hydrogen in the vacuum cavity increases the thermal conductivity across the vacuum cavity of the catalytic converter, thus assisting in removing heat to prevent overheating and damage to the catalytic materials.
However, passive thermal energy storage systems and the variable insulation systems have problems. In passive thermal energy storage systems, the thermal energy storage material is well-connected thermally to the catalytic converter so that heat will readily flow from the thermal energy storage material to the catalytic converter when it requires heat. However, when the converter has sat for a long time and the thermal energy storage material has cooled, this close coupling will draw heat from the hot exhaust gas stream and from the catalyst until the thermal energy storage material itself is heated to a high temperature. This will require a particularly long time during which untreated exhaust gases will be emitted. Thus, while the above-discussed design is effective in reducing “cold start” emissions when it is hot, it actually exacerbates the “cold start” problem whenever the converter has been allowed to cool. Notably, no matter how effectively the converter stores heat, there will be times when it has cooled and will suffer from some degree of “cold start” emissions. The electrically heated and controlled hydrogen source disclosed in Benson U.S. Pat. No. 5,477,676 to prevent the converter from overheating requires complex and expensive wiring to the catalytic converter. This wiring and control system also adds to the vulnerability of the exhaust system to failure and increases the risk of a warranty liability or expensive model recall action by the manufacturer. This can be a serious problem, particularly given the severe environments that exhaust systems are subjected to.
Yet another problem with vacuum-insulated catalytic converters is that it is difficult to tell if the vacuum exists, after the system is put on a vehicle and subjected to use. It is known to use a getter material to help preserve a vacuum. Getter materials combine with gases to form solids (or absorb gases into their atomic matrix) which helps maintain a sufficient vacuum in a sealed chamber. However, getter materials have a maximum absorbence value after which they are no longer effective. A catalytic converter structure is desired that facilitates placing a getter material within a vacuum cavity in a catalytic converter without using up the getter material prior to drawing a high vacuum in the cavity. It is desirable to seal a vacuum cavity prior to activating a getter material. But once a cavity is sealed, there is no economical or easy access to the vacuum cavity to allow measurement of the vacuum level.
Particulate traps, often used on diesel exhaust emissions, have similar problems to catalytic converters, in that they are most effective when at a minimum operating temperature. For example, vacuum-insulated structures are desired, including variable thermal insulative control, reduced thermal loss at low temperature and after engine shut-off, vacuum detection/indication after assembly, and the like. Another problem of particulate traps is their high weight.
Accordingly, exhaust treatment devices/structures are desired solving the aforementioned problems and offering the aforementioned advantages.